distinct in selection formula

StrSelFor = "{ip_stlmt_dtl.rev_dt}>=date(" + StrFyr + "," + StrFmnth + "," + StrFday + ") and {ip_stlmt_dtl.rev_dt}<=date(" + StrTyr + "," + StrTmnth + "," + StrTday + ") and {iptrn.chrg_cd}=99 and {iptrn.bil_no}={ip_stlmt_dtl.bill_no}"

in the above formula last condition is of {iptrn.bil_no}={ip_stlmt_dtl.bill_no}
in ip_stlmt_dtl there can be same bill_no more then once but i want to have only once in the report that should be distinct bill_no
how should i set the selectio forumula

in my report every record is repeated again.but i just want it once so how can i deal with that?